Induction heating apparatus

ABSTRACT

A circuit for supplying a series of current pulses to an output coil, such as a coil used for induction heating, can be constructed utilizing a timing circuit adapted to be connected to a DC power supply and a resonant circuit including the output coil connected to the timing circuit. The timing circuit used includes a current discharge or release means such as an SCR (silicon-controlled rectifier) and a trigger means such as a trigger diode (bidirectional diode thyristor) for actuating or firing the release means in response to current supplied by the power supply so that the release means will supply current to the resonant circuit. The current supplied to the resonant circuit results in the development of a resonant current in opposition to the supplied current. Such resonant current feeds back to the timing circuit so as to render the release means no longer operative to supply current to the resonant circuit until such time as thereafter the trigger means, as a result of power supplied to it from the power supply, actuates the release means so that it will again supply current to the resonant circuit.

United States Patent Cunningham [4 1 Jan. 25, 1972 154] INDUCTIONHEATING APPARATUS [57] ABSTRACT [72] Inventor: Ronald .I. Cunningham,4360 Eagle Rock A circuit for supplying a series of current pulses to anoutput View Drive, Los Angeles, Calif. 90041 coil, such as a coil usedfor induction heating, can be constructed utilizing a timing circuitadapted to be connected to a [22] Flled: July 1970 DC power supply and aresonant circuit including the output 2 APPL N 52,13 coil connected tothe timing circuit. The timing circuit used includes a current dischargeor release means such as an SCR (silicon-controlled rectifier) and atrigger means such as a [52] 219/10-75, 307/318 trigger diode(bidirectional diode thyristor) for actuating or [51] Int. Cl. ..H05b5/00 fi i the release means in response to current Supplied by he [58]Field of Search ..219/10.75;307/117, 129, 271, Power Supply so that therelease means will Supply current to 307/3 1 3 the resonant circuit. Thecurrent supplied to the resonant circuit results in the development of aresonant current in op- [56] Rem'enccs clued position to the suppliedcurrent. Such resonant current feeds back to the timing circuit so as torender the release means no UNITED STATES PATENTS longer operative tosupply current to the resonant circuit until 3,466,528 9/1969 Adams..321/45 such time as thereafter the gg r m n as a resul f power3,475,674 10/1969 Port fi ld t 1 ,321/45 supplied to it from the powersupply, actuates the release 3,339,126 8/1967 Horowitz ..317/ 124 meansso h i will gain supply curren to the resonant cir- 3,332,036 7/1967Kappenhagen et al. ..331/l73 cuit 3,021,413 2/1962 Blok ..219/10.753,187,204 6/1965 13 Claims, 7 Drawmg Figures Adkins ..307/1 17 PrimaryExaminer-J. V. Truhe Assistant Examiner-Gale R. Peterson Attorney-EdwardD. OBrian INDUCTION HEATING APPARATUS BACKGROUND OF THE INVENTION Theterm induction heating is commonly used to designate heating processesin which an object or charge of or containing one or more metals isheated by subjecting such an object or charge to an alternating magneticflux. Such a flux serves to induce rapidly reversing currents or eddycurrents in whatever is being heated by this type of process so as toresult in the generation of heat. Conventionally such an alternatingmagnetic flux is generated by passing an alternating current through acoil surrounding the object or charge being heated. Frequently, such acoil has to be cooled as by circulating water through it to avoid thecoil being melted.

The alternating currents used in this type of heating are normallygenerated utilizing conventional equipment for generating alternatingcurrents of the particular frequencies to be employed. In practice thefrequencies normally used vary in the range of from about 400 to about500,000 cycles per second. The precise frequency chosen for any specificinduction heating application will depend upon the nature of thatapplication. In general higher frequencies within the range indicatedare employed where it is desired to obtain essentially a surface typeheating. Thus, in so-called electronic or high-frequency inductionheating designed to heat primarily the outer surface or region of ametal object frequencies of from 100,000 to 500,000 cycles per second oreven higher are utilized.

An understanding of the present invention is not considered to require adetailed understanding of all of the problems which have beenencountered with conventional induction heating processes as are brieflyindicated in the preceding discussion. Frequently conventional inductionheating involves the use of radio frequencies. This entails or resultsin various problems which are familiar in the electronics industry. Withconventional induction heating processes power transmission problemshave frequently made it necessary or advisable to utilize specialtransmission cables and have frequently resulted in significant powerlosses. Also with conventional induction heating processes both highandlowfrequency components of an applied alternating current have beenequal, making it impossible to obtain simultaneously heating effects atdifferent depths within an object or charge being heated.

SUMMARY OF THE INVENTION An objective of the present invention is toprovide new and improved induction heating. More specifically anobjective of the present invention is to provide relatively simple,relatively inexpensive, relatively efiective circuits for use ininduction heating. A further objective of the invention is to supplycircuits which provide unique current pulses to an output coil, such asa coil used for induction heating, which pulses are especially useful ininduction heating.

It is not to be assumed from these objectives that such circuits areonly useful in induction heating. They can be employed in otherapplications where ever it is desired to supply a series of timedcurrent pulses to an output coil. Another objective of the presentinvention is to provide a new process for induction heating utilizingwhat may be regarded as both high and low frequency components so as tosimultaneously obtain heating effects at different depths within anobject or charge being heated.

In accordance with this invention these and various related objectivesare achieved through the utilization of a circuit for supplying a seriesof current pulses to an output coil such as the coil used for inductionheating. Such a circuit in accordance with this invention includes atiming circuit adapted to be connected to a DC power supply and aresonant circuit including the outp u t coil connected to the timingcircuit. The timing circuit employed includes a current discharge orrelease means such as an 'SCR (silicon-controlled rectifier) and atrigger means such as a trigger diode (bidirectional diode thyristor)and actuating the current release means so as to supply current to theresonant circuit.

In a circuit of this invention the current supplied to the resonantcircuit results in the development of a resonant current in oppositionto the supplied current which feeds back to the release means so as torender the release means no longer operative to discharge current to theresonant circuit until thereafter the trigger means, as a result of thepower supplied to it from the power supply, actuates the release meansso that it will a again supply current to the resonant circuit.

With the circuit of this invention the rate at which pulses are suppliedto the output coil in the resonant circuit can be varied and the shapesof such pulses can be varied to a limited extent as may be desired forvarious specific applications. With a circuit in accordance with thisinvention the current pulses supplied to the output'coil used have awaveform including positive and negative: components of differentmagnitudes. Pulses with such waveforms are considered quite advantageousin enabling circuits of the invention to be used in induction heating toheat an object or charge at different depths.

BRIEF DESCRIPTION OF THE DRAWINGS Further details of the presentinvention as well as the manner in which the invention achieves theaforegoing objects and other objectives and advantages of the inventionwill be apparent from a detailed consideration of the remainder of thisspecification, the appended claims and the accompanying drawings inwhich:

FIG. 1 is a schematic wiring diagram of a presently preferred embodimentor form of a circuit in accordance with this invention;

FIGS. 2 to 5 are curves showing current pulse waveform shapes at variouspoints in the circuit shown in FIG. 1 during the operation of thiscircuit;

FIG. 6 shows a modification of the circuit shown in FIG. 1 so as toutilize a feedback means in order to govern or control the pulserepetition rate of pulses generated in an output coil used in a circuitof the type shown in FIG. 1;

FIG. 7 is a modified schematic wiring diagram of a modified embodimentor form of a circuit in accordance with this invention. i

From a detailed consideration of the drawings and of the remainder ofthis specification it will be realized that the various circuits shownembody the concepts of the present invention, but are not in a technicalsense the invention itself. The basic concepts or principles of thepresent invention can be embodied within other somewhat differentcircuits through the use of routine electronic design skill.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings thereis shown a presently preferred form or embodiment of a circuit 10 ofthis invention which is primarily intended for use in induction heating,but which can be used for other purposes. This circuit 10 can beconsidered as being composed of two subcircuits, a timing circuit I2 anda resonant circuit 14. A significant feature of this invention is thesimplicity of these subcircuits.

The timing circuit 12 is composed of an SCR (silicon-controlledrectifier) 16, a trigger diode (bidirectional diode thyristor) abreak-back semiconductor) 18, a capacitor 20 and a variable resistor 22.These components are connected by means of wires 24 so that both oneterminal of the resistor 22 and the base of the SCR 16 can receive 8+power from an appropriate power source (not shown) through an input wire26, so that the other terminal of the resistor 22 is connected to oneside of the capacitor 20 and to one side of the trigger diode 18, sothat the other side of this diode 18 is connected to the gate of the SCRl6 and so that the other side of the capacitor 20 is connected to thecathode of the SCR 16.

The power source used may be any convenient DC power source. If desiredpower may be supplied to the input wire 26 through a mechanical orelectronic timing means used to determine the period of operation of thecircuit 10. The particular wire 24 connecting the capacitor 20 and thecathode of the SCR [6 is connected to a connecting wire 28 used tosupply power from the timing circuit 12 to the resonant circuit 14.

This resonant circuit 14 is a tank-type LC resonant circuit including acomparatively large inductance coil 30, a capacitor 32, and a work coil34 of significantly less inductance .than the coil 30. These componentsin the circuit 14 are connected by wires 36 between the connecting wire28 and a ground wire or connection 38 so that the inductance coil 30forms one leg of the circuit 14 and so that the capacitor 32 and thework coil 34 are connected in series so as to constitute the other legof this circuit 14. Preferably this circuit 14 is constructed so as toinclude terminals 40 useful in connecting the work coil 34 into thecircuit at a significant distance from the other components of thiscircuit 14.

When the circuit is to be used DC power is supplied through the wire 26to both the resistor 22 and the anode of the SCR [6. At the moment ofinitial use of this circuit 10 this SCR 16 will be in a nonconductivestate or condition and so will not pass the applied current. The appliedcurrent will, however, pass through the resistor 22 to both thecapacitor 20 and the diode 18. The resistor 22 serves a significantfunction as current is applied in protecting the diode 18 from thedirect application of 8+ current since if such current were applieddirectly to the diode 18 it would automatically go to its conductivestate or condition.

As current reaches the capacitor 20 it will become charged,

but the diode 18 will not pass the current applied to it until thecapacitor 20 is charged to what is referred to as the breakback voltageof this diode 18. When the capacitor 20 is charged, to this point, thediode 18 will go into what may be considered as a conductive state andwill allow current to pass to the gate of the SCR 16. As the diode 18goes into this conductive state it will continue to allow current topass at a lessor voltage than was required to render it conductive, andthe capacitor 20 will be discharged through the diode 18 to the gate ofthe SCR 16.

The .current which passes to the gate of the SCR 16 will render the SCRI6 conductive so that it releases" or discharges" or passes" the appliedB+ current from its anode to its cathode. The current released by theSCR 16 as a result of this will flow to the resonant circuit l4 throughthe wire 24 attached to the base of the SCR l6 and the wire 28. Becauseof the function of the assembly of the resistor 22, the trigger diode l8and the capacitor 20, as connected together they may be regarded as atrigger circuit" 42 for causing the release of current through or by theSCR l6 and may be termed a trigger means for this purpose. Because ofthe function of the SCR 16 it may be considered as a release means forreleasing or passing current applied to it in response to a signal fromthe trigger circuit 42.

The current passed by the SCR l6 reaching the resonant circuit 14 willflow in what may be regarded as an expected manner. Because of theinductance of the coil the capacitor 32 will be charged as current isreleased to the resonant circuit 14, and charging current will passthrough the work coil 34. This resonant circuit 14 will resonate inresponse to the applied current so as to give rise to a resonant currentin opposition to the B+ current released to or supplied to the resonantcircuit 14. The resonant current created in this manner will flow backto the timing circuit 12 through the wire 28 and the wire 24 connectedto the cathode of the SCR 16. When the resonant current moving in thismanner approximates the applied B+ voltage it will act in opposition tothe current flowing through the diode l8 and flowing through the SCR 16so as to cause the diode 18 to no longer pass a current to the SCR l6and so as to render the SCR l6 nonconductive. By this it is meant thatthe resonant current created as described will result in no more B+current being supplied to the resonant circuit 14 through the SCR 16.

At this point in time the circuit 10 will perform as when power isinitially supplied to the timing circuit 12. In other words thecapacitor 20 will become charged to the break-back voltage of the diodel8, and then this diode 18 will allow current to pass to the gate of theSCR 16. This will result in another pulse of current passing to thetiming circuit 14. As this occurs a further pulse of current will flowacross the work coil 34 and a pulse of resonant current will bedeveloped which will again prevent the applied B+ current flowing to theresonant circuit 14. The whole cycle of operation will continue again.The rate at which these operation cycles continue can be varied byadjusting the variable resistor 22. However it is noted that this rateis limited by the speed at which the SCR 16 can be switched betweenconductive and nonconductive states to below what is normally considereda radio frequency range.

A satisfactory circuit as shown in FIG.-l can be constructed for usewith a 8+ power supply of from 5 to 100 volts by using the followingcomponents: resistor 22-l 0 to 250 K/ohms, SCRl6General Electric SCRC141; capacitor 32-20 MFD, coil 30-20MH; work coil 34-15-25 micro H.When the circuit 10 is constructed with such components the pulses ofcurrent passing in the circuit will have various wave forms as indicatedin FIGS. 2-5 of the drawings during each cycle of operation. At thepoint A, the current used to charge the capacitor 20 and which flowsthrough the diode 18 will have a wave shape as indicated in FIG. 2. Atthe other side of this capacitor 20 where this capacitor 20 is connectedto the resonant circuit 14 at point B the current wave shape will be asshown in FIG. 3. The wave shape of the current at the capacitor 32 atpoint C is shown in FIG. 4. Most important, however, the waveform of thepulses across the work coil 34 as shown in point D in FIG. I are shownin FIG. 5 of the drawings.

This shape of a current pulse as developed across the work coil 34 asshown in FIG. 5 is considered quite important to this present invention.It will be noted that a current pulse as shown has both positive andnegative components of different magnitudes. The difference in theamplitude of these two components is considered to give rise todifferent heating effects at different depths in a charge or objectbeing heated by the work coil 34 when the circuit 10 is used forinduction heating. For certain applications this is advantageous. Thisis to be contrasted with prior induction heating processes in which thepositive and negative values of the current pulses applied to a workcoil used in induction heating have been equal or nearly equal. Suchprior processes have been of such a nature that they have tended to heatto or at substantially a single depth.

The invention is also significantly advantageous for use in inductionheating because the circuit 10 can be used employing a work coil 34 ofvirtually any desired shape. This work coil 34 may be standardcylindrical coil. It may also be a flat spiral like coil. It may also besuch a spiral like coil bent in a U or other analogous shape so as tofit around a part of an object being heated. Because of the fact thatthe coil 34 may have many different shapes, the circuit 10 is especiallyadvantageous for use with many induction heating applications where inthe past induction heating could not be satisfactorily employed.

The circuit 10 is also considered quite advantageous because of the factthat it does not use radio frequencies. Thus, when the circuit 10 isused for induction heating problems such as radio frequency burnsencountered with prior related induction heating apparatus are notencountered. Because of the nature of the current pulse applied to thework coil 34, these pulses can be easily transmitted across significantdistances without significant power losses. This is frequentlyconsidered quite beneficial and advantageous.

In FIG. 6 of the drawings there is shown a modified circuit of thepresent invention which is essentially very similar to the circuit 10shown in FIG. 1. In the interest of brevity various parts of the circuit100 which are the same or substantially the same as corresponding partsof the circuit 10 are not separately described herein and are indicatedin the drawings and where necessary for explanatory purposes in thespecification by the numbers previously used to indicate such partspreceded by the number 1.

The circuit 100 utilizes a feedback circuit or means 50 in order toaccurately control the frequency of the current pulses across the workcoil 134.ln order to accomplish this, it employs a coil 52 which iscoupled magnetically to the coil 130 by what may be termed astransformer-type coupling. Wires 54 are employed to connect the ends ofa potentiometer 56 across the ends of the coil 52. Other wires 58connect a lightbulb 60 between one end of the coil 52 and the wiper onthe potentiometer 56. With this circuit 50 the lightbulb 60 is locatedopposite a conventional light-sensitive resistor 122 used to replace thevariable resistor 22 in the circuit 10.

The operation of the circuit 100 is essentially very similar to theoperation of the circuit 10. The timing circuit 112 and resonant circuit114 in the circuit 100 operate as in the circuit 10. When, however,pulses of current pass through the coil 130, these induce a voltage inthe coil 52. The induced voltage in the coil 52 is used so as to causeillumination of the lightbulb 60 in accordance with the frequency of thepulses flowing in the resonant circuit 14. Variations in theillumination of the lightbulb 60 in turn effect the resistance value ofthe resistor 122 and this in turn results in a variation in the pulserepetition rate at which current is passed by the SCR 116 as when thevariable-resistor 22 is adjusted in the circuit 10.

With this circuit 100 asthe frequency applied to the work coil 134decreases, the voltage in the coupled coil 52 increases so as toincrease the intensity of light from the lightbulb 60. Such increase inthe intensity of the light from the lightbulb 60 will serve to decreasethe resistance of the resistor 122, resulting in an increase in thepulse repetition rate or frequency applied to the work coil 134. This inturn will result in a decrease in the intensity of the light by the bulb60 as the result of a manner of operation as indicated.

In FIG. 7 of the drawings there is shown a still further modifiedcircuit 200 of the present invention which in effect is two of thecircuits 10 as shown in FIG. 1. In the interest of brevity, variousparts of the circuit 200 which are the same or substantially the same ascorresponding parts of the circuit 10 are not separately describedherein and are indicated in the drawings and where necessary forexplanatory purposes, in this specification by the numbers previouslyused to indicate such parts preceded by the number 2.

In the circuit 100 the timing circuit 212 indicated at the right of FIG.7 of the drawings is identical with the timing circuit 12 previouslyindicated except for the fact the a wire 224 connected to the variableresistor 222 is connected to the wire 228 instead of being connecteddirectly to a source of 8+ power. In this timing circuit 212 shown atthe right of FIG. 7 the base of the SCR 216 is connected to the wire 226which is used to lead to a source of 3+ power.

Both of the timing circuits 212 shown in FIG. 7 of the drawings in thecircuit 200 operate in the same manner as the timing circuit 12previously described with one difference. The timing circuit 212 asshown at the right hand side of FIG. 7 includes a trigger circuit 242which is charged so as to permit the associated SCR 216 to pass B+current by current pulses passed by the SCR 16 in the timing circuit 202at the left of FIG. 7. This will result in the sequential operation ofthe two timing circuits 212, the second or right hand circuit beingoperated in response to operation of the first timing circuit 212. Withthis structure this will result in the work coil 234 at the left of FIG.7 receiving a current pulse as for induction heating and then the workcoil 234 at the right of FIG. 7 receiving a current pulse for the samepurpose at a subsequent period.

The circuit 200 is normally used with the two work coils 234 in closeproximity or intertwined with one another. It is useful in avoidingmagnetization effects in an object or charge being heated by inductionheating. The circuit 200 is also considered advantageous inasmuch as itapplies relatively frequent current pulses through the two work coils234 in induction heating. In general, the more rapid the pulserepetition is in such heating the more rapidly an object or charge isheated.

From a careful consideration of the preceding it will be realized thatmany variations may be made in the precise circuits shown through theuse or application of routine electronic skill. Thus, for example,different feedback means than the precise feedback circuit described maybe employed. Various circuits as indicated may be operated in parallelwhere this is desired. In all these circuits as herein indicated, thecomponents used must be optimized with respect to one another to achievea pulse repetition rate as desired for a particular application. It willbe realized that different rates will be desired for various differentapplications to which circuits as described may be put.

I claim:

1. A circuit for supplying a series of timed current pulses to a workcoil which comprises:

a timing circuit means for receiving a DC current from a power sourceand for supplying current pulses of such power,

said timing circuit means including release means for releasing currentfrom said power source and trigger means for causing the release ofcurrent by said release means in response to power from said powersource,

said trigger means being connected to said release means,

said release means and said trigger means both being connected to saidpower source,

said release means being responsive to a current supplied to it inopposition to a current released through it from said power source so asto no longer release current from said power source,

a resonant circuit means for receiving current released by said releasemeans connected to said release means so as to receive such current,said work coil forming a part of said resonant circuit means,

said resonant circuit means being capable of developing a resonantcurrent in opposition to the current released to it by said releasemeans, which resonant current will be fed back to said release means tocause said release means to no longer current to said resonant circuitmeans until said trigger means again actuates said release means.

2. A circuit as claimed in claim 1 wherein:

said release means is a silicon-controlled rectifier.

3. A circuit as claimed in claim I wherein:

said trigger means is a trigger circuit including a resistor, acapacitor and a trigger diode, said resistor, said capacitor and saiddiode being connected so that said capacitor is charged by current fromsaid power source which has passed through said resistor to a sufficientextent to cause said diode to become conductive, said diode beingconnected to said release means so as to cause actuation of said releasemeans when said diode becomes conductive and passes a current.

4. A circuit as claimed in claim 1 wherein:

said resonant circuit is an LC tank circuit having inductance of one legthereof and having a capacitance and said work coil in the other legthereof.

5. A circuit as claimed in claim I wherein:

said release means is a silicon-controlled rectifier,

said trigger means is a trigger circuit including a resistor, acapacitor and a trigger diode, said capacitor and said diode beingconnected so that said capacitor is charged by current from said powersource which has passed through said resistor to a sufficient extent tocause said diode to become conductive, said diode also being connectedto the gate of said silicon-controlled rectifier so as to pass a currentto said silicon controlled rectifier when said diode becomes conductive,

the base of said silicon-controlled rectifier is adapted to be connectedto said power source,

the cathode of said silicon-controlled rectifier and said capacitor areconnected to said resonant circuit means,

said resonant circuit means is an LC tank circuit having an inductancein one leg thereof and having a capacitance and said work coil in theother leg thereof.

6. A circuit as claimed in claim 5 wherein:

8. A circuit as claimed in claim 7 including: a photosensitive resistorconnected to said trigger means so that power to said trigger meanspasses through said resistor and wherein,

said feedback means includes means for receiving power from saidresonant circuit means coupled to said resonant circuit means and anillumination source operated thereby, said illumination source beinglocated so that the illumination therefrom controls the resistance ofsaid resistor.

9. A circuit as claimed in claim 1 including:

a second of said timing circuit means and a second of said resonantcircuit means connected thereto,. said second resonant circuit meansincluding another work coil,

said trigger means of said second timing circuit means being connectedto said release means of the first-mentioned timing circuit means so asto be actuated thereby, allowing the release means of the second timingcircuit means to release a current in response to current released bysaid release means of said first-mentioned timing circuit means.

10. In an apparatus for induction heating in which a current from apower supply is passed to a work coil through intermediate circuitry theimprovement which comprises:

said intermediate circuitry consisting essentially of a timing circuitmeans and a resonant circuit means, said timing circuit means beingconnected to said power supply and to said resonant circuit means sothat power is supplied to said resonant circuit means through saidtiming circuit means, said resonant circuit means including said workcoil as a part thereof,

said timing circuit means being capable of releasing a current tosaidresonant circuit means,

said resonant circuit means being capable of resonating in response tocurrent released to it through said timing circuit means so as todevelop a resonant current in opposition to the current supplied to itthrough said timing circuit means,

said timing circuit means being responsive to the resonant currentdeveloped in said resonant circuit means so as to no longer supply acurrent to said resonant circuit means until said timing circuit meansis actuated so as to release current to said resonant circuit meansbecause of power supplied to it from said power supply.

11. A process of induction heating in which current from a power supplyis used to supply power to a work coil in which the improvementcomprises:

using said power supply to supply current to actuate a discharge meansso that it will pass the current supplied to it to a resonant circuitincluding said work coil, causing said resonant circuit to develop aresonant current in opposition to the current passed by said dischargemeans and thereafter using said resonant current to render saiddischarge means incapable of passing a current from said power supplyuntil said discharge means is again actuated by current supplied by saidpower supply and automatically repeating the steps herein specified atintervals as determined by the circuit components employed.

12. A process as claimed in claim 13 wherein:

said discharge means is a silicon-controlled rectifier and saiddischarge means is actuated by a trigger means connected to saidsilicon-controlled rectifier and to said power supply, said triggermeans also being connected to said resonant circuit.

13. A process as claimed in claim 11 wherein:

said current pulses have a wave shape as indicated in FIG. 5

of the drawings.

and that said Letters Patent are hereby cortected as shown below:

' 222%? I UNITED STATES ,PATENT OFFICE v I CERTIFICATE OF CORRECTION Patent NQ'. 3,;637 .970 ed January 25., 1972 ve VRONALD J CUNNINGHAM It iscertified that error appears in the above-identified patent In theclaims, claim 12 should be dependent on Claim 11 t Signed and seeledthis 10th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents 2 22 3 UNITED STATES PATENT OFFICE RTIFICATE 0F CORREQTONPatent No. 3 ,637 ,970 Dated January 25, l972 Inventor( LD J. CUNNINGHAMIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In the claims, claim l2 should be dependent on claim ll Signed andsealed this 10th day of October 1972.

(SEAL) Attest:

EDWARD MlFLETCHEmJR. ROBERT GOTTSCHALK Attesting Officer Commissioner ofPatents

1. A circuit for supplying a series of timed current pulses to a workcoil which comprises: a timing circuit means for receiving a DC currentfrom a power sOurce and for supplying current pulses of such power, saidtiming circuit means including release means for releasing current fromsaid power source and trigger means for causing the release of currentby said release means in response to power from said power source, saidtrigger means being connected to said release means, said release meansand said trigger means both being connected to said power source, saidrelease means being responsive to a current supplied to it in oppositionto a current released through it from said power source so as to nolonger release current from said power source, a resonant circuit meansfor receiving current released by said release means connected to saidrelease means so as to receive such current, said work coil forming apart of said resonant circuit means, said resonant circuit means beingcapable of developing a resonant current in opposition to the currentreleased to it by said release means, which resonant current will be fedback to said release means to cause said release means to no longercurrent to said resonant circuit means until said trigger means againactuates said release means.
 2. A circuit as claimed in claim 1 wherein:said release means is a silicon-controlled rectifier.
 3. A circuit asclaimed in claim 1 wherein: said trigger means is a trigger circuitincluding a resistor, a capacitor and a trigger diode, said resistor,said capacitor and said diode being connected so that said capacitor ischarged by current from said power source which has passed through saidresistor to a sufficient extent to cause said diode to becomeconductive, said diode being connected to said release means so as tocause actuation of said release means when said diode becomes conductiveand passes a current.
 4. A circuit as claimed in claim 1 wherein: saidresonant circuit is an LC tank circuit having inductance of one legthereof and having a capacitance and said work coil in the other legthereof.
 5. A circuit as claimed in claim 1 wherein: said release meansis a silicon-controlled rectifier, said trigger means is a triggercircuit including a resistor, a capacitor and a trigger diode, saidcapacitor and said diode being connected so that said capacitor ischarged by current from said power source which has passed through saidresistor to a sufficient extent to cause said diode to becomeconductive, said diode also being connected to the gate of saidsilicon-controlled rectifier so as to pass a current to said siliconcontrolled rectifier when said diode becomes conductive, the base ofsaid silicon-controlled rectifier is adapted to be connected to saidpower source, the cathode of said silicon-controlled rectifier and saidcapacitor are connected to said resonant circuit means, said resonantcircuit means is an LC tank circuit having an inductance in one legthereof and having a capacitance and said work coil in the other legthereof.
 6. A circuit as claimed in claim 5 wherein: said resistor is avariable resistor.
 7. A circuit as claimed in claim 1 wherein: feedbackmeans coupled to said resonant circuit means and operatively connectedto said trigger means for controlling the operation of said triggermeans in response to current pulses in said resonant circuit means sothat the frequency of current pulses in said resonant circuit meansremains constant.
 8. A circuit as claimed in claim 7 including: aphotosensitive resistor connected to said trigger means so that power tosaid trigger means passes through said resistor and wherein, saidfeedback means includes means for receiving power from said resonantcircuit means coupled to said resonant circuit means and an illuminationsource operated thereby, said illumination source being located so thatthe illumination therefrom controls the resistance of said resistor. 9.A circuit as claimed in claim 1 including: a second of said timingcircuit means and a second of said resonAnt circuit means connectedthereto, said second resonant circuit means including another work coil,said trigger means of said second timing circuit means being connectedto said release means of the first-mentioned timing circuit means so asto be actuated thereby, allowing the release means of the second timingcircuit means to release a current in response to current released bysaid release means of said first-mentioned timing circuit means.
 10. Inan apparatus for induction heating in which a current from a powersupply is passed to a work coil through intermediate circuitry theimprovement which comprises: said intermediate circuitry consistingessentially of a timing circuit means and a resonant circuit means, saidtiming circuit means being connected to said power supply and to saidresonant circuit means so that power is supplied to said resonantcircuit means through said timing circuit means, said resonant circuitmeans including said work coil as a part thereof, said timing circuitmeans being capable of releasing a current to said resonant circuitmeans, said resonant circuit means being capable of resonating inresponse to current released to it through said timing circuit means soas to develop a resonant current in opposition to the current suppliedto it through said timing circuit means, said timing circuit means beingresponsive to the resonant current developed in said resonant circuitmeans so as to no longer supply a current to said resonant circuit meansuntil said timing circuit means is actuated so as to release current tosaid resonant circuit means because of power supplied to it from saidpower supply.
 11. A process of induction heating in which current from apower supply is used to supply power to a work coil in which theimprovement comprises: using said power supply to supply current toactuate a discharge means so that it will pass the current supplied toit to a resonant circuit including said work coil, causing said resonantcircuit to develop a resonant current in opposition to the currentpassed by said discharge means and thereafter using said resonantcurrent to render said discharge means incapable of passing a currentfrom said power supply until said discharge means is again actuated bycurrent supplied by said power supply and automatically repeating thesteps herein specified at intervals as determined by the circuitcomponents employed.
 12. A process as claimed in claim 13 wherein: saiddischarge means is a silicon-controlled rectifier and said dischargemeans is actuated by a trigger means connected to saidsilicon-controlled rectifier and to said power supply, said triggermeans also being connected to said resonant circuit.
 13. A process asclaimed in claim 11 wherein: said current pulses have a wave shape asindicated in FIG. 5 of the drawings.